1. Field of the Invention
The present invention relates to high strength, high toughness Bainitic Steels.
2. Background Art
Austempered Ductile Cast Iron (ADI) has emerged as a major engineering material in recent years because of its excellent properties, such as high strength with good ductility, good wear resistance, good fatigue properties, and good fracture toughness. ADI is widely used in manufacturing components such as gears, crankshafts, and locomotive wheels, connecting rods, brake shoes, and the like. ADI has other advantages, such as low production cost arising from its good castability, excellent machinability resulting in consequently longer tool life, and shorter heat treatment processing cycles.
ADI is an alloyed and heat-treated nodular cast iron. It has a unique acicular matrix structure that consists of high carbon austenite (γHC) and ferrite (α) with dispersed graphite nodules. This unique microstructure, the product of the austempering process, provides the excellent mechanical and physical properties of ADI.
During austempering, ADI goes through a two-stage phase transformation process. In the first stage, the austenite (γ) decomposes into ferrite (α) and high carbon austenite (γHC):γ→α+γHC  (1)If the casting is held at the austempering temperature for too long, a second reaction takes place, during which the high carbon austenite γHC can further decompose into ferrite and carbide:γHC→α+ε  (2)
Steels with bainitic structure have several advantages including high strength, high toughness, and high ductility. In conventional steels, generally, the fracture toughness decreases as yield strength increases. On the other hand, the fracture toughness will be high when the yield strength is low. Thus, the combination of high strength and high fracture toughness cannot easily be obtained in most conventional steels.
When steel is austempered at temperatures below the noise of the TTT (Time Temperature and Transformation) curve, a structure is produced in which ferrite and iron carbide are not lamellar. This transformation product is called bainite. Bainite in steel has a needle-like (acicular) microstructure. Whereas pearlite is nucleated by iron carbide and is accompanied by the subsequent formation of ferrite, bainite is nucleated by ferrite, followed by the precipitation of iron carbide. This process leads to the dispersion of iron carbide in a ferrite matrix. With a lower transformation temperature, the distribution of carbide is finer and the ferrite needles are thinner. This product is called lower bainite. The transformation product at relatively higher temperatures consists of lath or plate-shaped ferrite units arranged in packets and interlath carbide precipitates. This product is called upper bainite. In steel, lower bainite produces higher yield strength but lower toughness. While the austempering reaction in ADI is a two-step reaction process, it is a one-step reaction process in the case of steel. During austempering in steel, austenite directly decomposes into acicular ferrite and carbide (bainite). Austempering of steel offers the advantages of increased ductility, reduced distortion, and a short overall time cycle to harden thoroughly. Generally, upper bainitic temperatures are above 316° C. (600° F.) in the case of steels and cast iron. Temperatures between 232° C. (450° F.) to 316° C. (600° F.) are the lower bainitic temperature range.
Accordingly, there is a need for new steels simultaneously exhibiting high yield strength and high fracture toughness.